Very thin silicon wafer base solar cell

Abstract

The performance and ruggedness of very thin silicon back surface field (BSF) solar cells are improved by the formation of a relatively thick, epitaxially grown, highly doped layer at the back of the cell and the formation of an arsenic doped layer at the top of the cell within the phosphorous diffused front region. As a result of these modifications and the method used for fabricating the modified cell, highly effective barriers, which diminish mobile charge loss by recombination, are created at the front and back of the cell base. The cell, consisting of a high resistivity, high minority carrier lifetime, very thin base sandwiched between effective barriers, permits achievement of almost ideal performance and has improved radiation damage resistance. The ruggedness of the very thin base cell is due to the addition of the relatively thick epitaxial layer in back of the base.

What is claimed is: 
     
       1. An improved silicon back surface field solar cell comprising a high quality very thin silicon single crystal wafer base 0.0005 to about 0.0004 inch thick, said base containing a back surface field region of the same type, P or N, as said base, said back surface field region having a thickness of about 1 μm and a dopant surface concentration of about 10 19  atoms/cm 3 , said cell having a substantially thicker epitaxial layer of silicon, doped with an impurity concentration which will provide the desired surface impurity concentration for the diffused back surface field region, interposed between the back surface field region and the cell back contact, said interposed layer having a thickness sufficient to provide required cell ruggedness. 
     
     
       2. A solar cell, as claimed in claim 1, whose base is P type and which has a top shallow diffused region consisting of a phosphorus diffused layer at the top of which is an arsenic diffused layer, the arsenic surface concentration being about ten times that of the phosphorus surface concentration. 
     
     
       3. A cell, as claimed in claim 1, whose base is cut from a high quality silicon ingot having a resistivity of about 1.0 ohm-cm or greater. 
     
     
       4. A cell, as claimed in claim 1, whose base is cut from a silicon ingot having a minority carrier lifetime of 100 microseconds or greater. 
     
     
       5. A method for fabrication of a silicon back surface field solar cell having a very thin silicon single crystal wafer base 0.0005 to about 0.004 inch thick, comprising the steps of (a) epitaxially depositing a layer of silicon on the back surface of a comparatively thick single crystal silicon wafer, said deposited layer being doped during deposition so as to provide a required impurity and impurity surface concentration for creation of an optimum back surface field diffused region in said wafer, said diffused region being of the same semiconductor type, P or N, as said wafer and being formed in said wafer during said deposition process, said diffused region having an impurity surface concentration of about 1×10 19  atoms/cm 3  and a depth of about 1 μm, said deposited silicon layer thickness being such as to provide the ruggedness required for the completed cell, (b) thinning the resultant wafer by removing silicon from its top surface to obtain the required base thickness for the cell, (c) creating a shallow P-N junction at the top of said wafer, and (d) applying top and back contacts and a top surface antireflection coating.   
     
     
       6. The method, as claimed in claim 5, wherein the silicon wafer is cut from a high quality ingot having a resistivity about 1.0 ohm cm or greater. 
     
     
       7. The method, as claimed in claim 5, wherein the silicon wafer is cut from a high quality ingot having a minority carrier lifetime of 100 microseconds or more. 
     
     
       8. The method as claimed in claim 5, wherein the shallow P-N junction at the top of the cell is made by simultaneous diffusion of arsenic and phosphorus with conditions such that the arsenic surface concentration is about ten times that of the phosphorus surface concentration. 
     
     
       9. A method for fabrication of an improved, back surface field, very thin base, cell in which a wafer is cut from a P type, float-zone, zero dislocation density, silicon ingot having a resistivity of 1.0 ohm cm or greater, said wafer being thick in comparison to the thickness desired for the base, growing an epitaxial layer on one surface, the back, of said wafer, said epitaxial layer being doped, during its growth, with boron to a concentration of about 1×10 19  atoms/cm 3 , a boron diffused layer having a thickness of about 1.0 micrometer being formed in said wafer during the growth of said epitaxial layer, thinning said wafer to the thickness desired for the base by removing silicon from the surface opposite the epitaxial layer, the front surface, diffusing an N type impurity or impurities to form a shallow N + , P junction below the front surface, applying contacts to the top and bottom of the structure so formed, and applying an antireflection coating to the top surface. 
     
     
       10. The method, as claimed in claim 9, in which the shallow N + , P junction is formed by simultaneous diffusion of arsenic and phosphorus into the front surface, the depth of phosphorus diffusion being about 0.5 micrometers and the corresponding depth of arsenic diffusion being about 0.2 micrometers, the phosphorus surface concentration being limited to about 1×10 19  atoms/cm 3  and the arsenic surface concentration being about 1×10 20  atoms/cm 3 .